1. Field of the Invention
The invention generally relates to materials, methods and systems to create better material systems for forming interconnected or continuous layers from smaller constituent elements. These types of material systems can be employed in, for example, 3D printing or powder coating applications. More particularly, the present invention relates to methods and systems for performing deposition processes using plasma and the compositions of matter created.
2. Description of the Relevant Art
Powdered materials comprising active polymeric materials are used in a variety of industrial processes and products. These material types are used in composite materials as fillers, binding agents and in other functions. They can also be used to form interconnected or continuous material layers in powder coating or additive 3D printing processes where a layer of powder is first placed in a desired location and then processed, typically using some kind of heating method, to allow the powder particles to at least partially bond to form a continuous, though possibly porous, layer. For materials to perform well in these applications they need to have a variety of properties within certain ranges, including melt point, melt viscosity, enthalpy of formation, other chemical properties and particle features including size, size distribution, shape and surface roughness and other physical properties. For both powder coating and powder-based additive manufacturing, many combinations of materials have been used that include multiple polymers, flow agents, anti-oxidants, cross-linkers, coloring agents, reinforcing materials, flexible materials, insulating materials and other materials.
As solvent-free and environment-friendly coating systems, powder coating materials have acquired considerable importance and are preferred over solvent-borne coating materials in numerous fields of use. They often comprise binders, pigments, fillers and, where appropriate, additives and crosslinkers. They are in powdered or particulate form and are generally applied electrostatically to a large number of different substrates, on which they are typically processed by baking or by radiative energy. The powder coating materials must at least partially melt or soften during this heating process and form an interconnected or continuous layer and bond to the substrate.
There are a variety of additive technologies that can be used to build shapes from small, standard elements rather than formative approaches, such as molding, or subtractive approaches including machining. Other terms of art related to Solid Freeform Fabrication include ‘3D Printing’, ‘Additive Manufacturing’, ‘Rapid Prototyping’ and ‘Rapid Manufacturing’. The standard elements used in these processes can be thin filaments of material, powders, pastes or liquid elements. While some additive fabrication methods, such as Stereolithography, selectively cure portions of a material to form 3D objects, most additive processes use some kind of preferential heating. One commercial method for powder based additive manufacturing is selective laser sintering (“SLS”) a term of art that can refer to both solid state sintering and to partial or complete melting of at least a portion of the powder particles or certain materials comprising the powder particles. SLS is used herein as one example of many additive processes for which the present invention has valuable application. For SLS processing, viable materials include metals, polymers, ceramics and combinations thereof. Similar to powder coating materials they are at least partially melted during a heating process and form bonds or fuse to a substrate. Since multiple layers are bonded together in SLS processes, they represent more complex processes. Even so, guiding principles and limitations of the SLS process, the focus of the following discussion, generally apply to powder coating materials as well.
In order to produce a shaped article using a typical SLS process, powder is applied in a thin layer to the top of a chamber having a movable bottom surface. The chamber is in a larger enclosure, which typically reduces exposure of the powder to oxygen and is heated to a precisely controlled temperature slightly below the melting point of the powdered material. A laser, or other energy directing means, is used to heat at least an outer region of the particles in a desired portion of the powder layer to a temperature above the melt point of at least one component of the powdered material. The heated particles can then bond together, which can include at least a portion of the particles flowing together, to form a layer of the shaped article. The layer thickness is selected so that heating from the laser substantially propagates through the layer. In many currently available SLS machines, computer controlled minors or other laser aiming approaches steer a laser to heat desired regions of the top powder layer. After this step, the bottom surface of the power chamber is lowered by an amount corresponding to the layer thickness, often from 0.1-2 mm. The procedure is repeated by applying a fresh layer of powder. After the preselected number of these cycles have been completed, a volume has been filled in the chamber with the intended number of layers and consisting of unbonded powder volumes and bonded powder volumes corresponding to the desired shaped article. Unbonded powder regions stabilize the shaped article during and after processing.
More complete descriptions of this selective laser sintering technology may be found in U.S. Pat. Nos. 4,863,538; 5,132,143; 4,944,817; and 4,247,508, all of which are incorporated herein by reference.
While shaped articles have been produced by SLS processing in a variety of materials, few are available commercially. Commercial SLS materials provide dimensional accuracy and precision, relatively smooth surface finish, relatively high-speed processing, an ability to reuse un-bonded powder material more than once and costs that compete with more traditional fabrication routes. The ideal SLS material would match the dimensional tolerances and material properties of a molded or machined part and have superior costs when time, tooling, storage and the value of rapid part changes are also considered. In some cases, and especially for semicrystalline polymeric base materials, commercial sintering materials have more than one melt point, ‘Tm’:(a) a first Tm(‘Tm1’) when the material is melted a first time; and (b) a second Tm(‘Tm2’), which is lower than Tm1, when the material is melted or softened a second (or subsequent) time due to the material's transition from a crystalline to a more amorphous state. An SLS machine, comprising a powder chamber and an enclosure, as described above, can heat such material, and in particular the top layer of that material within the powder chamber, to a temperature below Tm1, yet near Tm2. Subsequent heating of a desired region of the powder layer by a laser or other means can melt at least a portion of the powder within those regions and melted elements of the powder can remain in a molten (or partially molten) state, without melting the remaining un-fused, or unbonded powder, because the melt point of the molten fraction of material then has a different melt point, T.m2. It is also possible for the molten fractions of powder to remain in at least a partially molten state while the subsequent layer is applied above it. Melting in the subsequent layer, combined with at least partially molten material in the previous layer allows for inter-layer bonding and can reduce stresses within the complete shaped object increasing the dimensional accuracy of the shaped article and precision of multiple shaped articles. Even so, shaped articles remain anisotropic. As an example, the strength (stiffness, elongation, peak stress, etc.) of shaped articles are typically higher in the plane of the powder layers (X and Y direction) than across multiple powder layers (Z direction). Higher part bed temperatures (the temperature at which the partially melted or sintered powder and the unmelted or unsintered powder is held) help with better adhesion in the Z direction and help relieve internal stresses which deform parts caused by formed layers cooling too quickly.
Titanium, iron, nickel and aluminum-based alloys have been demonstrated in what is called “Direct Metal Selective Laser Sintering”. Metals do not typically have a Tm1 and Tm2, but do have relatively high thermal conductivity and heats of enthalpy. The heating of an upper layer tends to induce at least some melting in the previous layer to a greater extent than processes focused on polymeric materials. Machines used to process metals run at higher temperatures, have more carefully controlled gas environments and higher laser powers when compared to machines designed to process polymeric materials. Still, the basic idea that a loose powdered material needs to co-exist in close proximity with a partially molten version remains for metallic materials in SLS.
The density and interlayer bonding within a shaped article are partly determined by the melt rheology of the molten portion of the powdered materials. Ideally, a shaped article produced via SLS would approach the density of an unpowdered, solid volume of the material. Low viscosities are typically related to higher density, closer to a solid part from a lower powder density, and to stronger parts, due to improved interlayer bonding. Melt viscosity of molten materials tends to fall with increasing temperature. The temperature that can be reached is limited by the laser or other heating means used, by a desire to perform the heating rapidly and by a desire to limit heating to regions that are desired to become part of the shaped article. A commercial SLS material must reach a sufficiently low melt viscosity within a bounded temperature increase.
Further, higher enthalpy of fusion in the powdered material corresponds to lower geometric tolerances of the shaped article. Materials having lower enthalpies of fusion tend to exhibit bonding between particles outside of desired regions, since heat conducts beyond the regions heated by the laser or other energy addition means. This effect also tends to reduce the ability to reuse materials in the powder chamber that do not become a part of shaped articles.
Since SLS powders are held at elevated temperatures, thermal degradation of the materials can occur, limiting the desired properties of the shaped articles and the recyclability of the powder. The formation of shaped articles can take several hours in an SLS process, exposing powdered materials to elevated temperatures for significant periods of time. Anti-oxidant materials are often mixed into SLS powders and low-oxygen atmospheres are maintained in SLS enclosures to counteract these effects. Degradation, in terms of SLS materials, can also refer to changes to the molecular structure of a material, surface or bulk chemical reactions beyond those involving oxygen or other changes, whether or not they are enhanced by aging or exposure to elevated temperatures, where the changes to an SLS material reduce the effectiveness of SLS in creating viable parts.
Physical properties of the powdered material particles are also important in current SLS technologies. A powdered material will have a density determined in part by powder particle size, particle size distribution, particle sphericity, particle surface roughness. Larger particles and tighter size distributions generally increase powder density as do lower roughness and higher sphericity. ‘Flow agents’, such as fumed silica are mixed into SLS powders to reduce inter-particle friction and other physical interaction which allows the powders to ‘flow’ to higher densities within the powder chamber.
The number of materials that can support the commercial SLS production of shaped articles, that meet melt rheology, complex melt point, enthalpy of fusion, thermal degradation and physical characteristics, remain very limited. For this reason the types of shaped articles and the applications they can address are also limited. Similarly, material options for powder coating are also limited. In these and other related application a much larger set of materials is desired.
Monsheimer, Sylvia et al. in U.S. Patent Application Publication No. 2004/0102539 propose how using a polyamide having an excess of carboxy end groups can be beneficial in SLS. They demonstrate that the bulk chemical composition of the polyamide can improve the recyclability of a powder when used in SLS production.
Barlow et al. in U.S. Pat. No. 6,048,954 highlights some of the benefits that can be achieved by coating powered materials for SLS, which in their case are metal or ceramic powders. They use lower melting and lower molecular weight coatings as binders to hold the particles together long enough to fuse or sinter the parts in a post process after which the binder is mostly calcined or baked off. Others in U.S. Pat. Nos. 5,076,869, and 6,814,926 similarly coat or blend metal particles with a polymer which is used to hold the coated powders together after an SLS process until they can be further fused or reinforced with a metal infiltration process. Monsheimer, Sylvia et al. in U.S. Pat. No. 7,988,906 demonstrate a benefit of coating materials which have melting points too high to run in normal SLS machines with cyclic oligomers. The cyclic oligomer rings are then opened and polymerized to hold the coated materials together. Preifer et al. in U.S. Patent Application Publication No. 2006/0159896 coat powdered particles for use in SLS with a material that has a softening point lower than 70° C. (closer to room temperature) to help bond the particles at lower bed temperatures. Preifer et al. in U.S. Patent Application Publication No. 2006/0251535 take this coating concept further by using an activatable adhesive as the material which coats the powdered particles. These methods all rely on the coating material to be the active material that holds or glues the particles together in order to make a part or to form a part that can be further processed and/or hardened.